Turbine device for hot air generation

ABSTRACT

The pressure head needed to drive combustion air into the combustion region (46) of a heat gun (930) is imparted to the air by a compressor blades (36) driven by a turbine whose blades (32) are interposed in the fuel conduit that leads to the combustion region (46). The expansion of the gaseous fuel, which is obtained from a pressurized source, provides the power to impose the necessary pressure head. The turbine-compressor combination is efficient enough that it can extract from the fuel flow enough power to maintain the necessary pressure head not only on the combustion gas (A 1 ), which flows through vents (44) in a manifold (43), but also on a larger volume of air (A 2 ) delivered downstream of the combustion region (46) to dilute the combustion products and thereby cool them to the desired heat-gun output temperature.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.08/045,822 filed on Apr. 9, 1993, now U.S. Pat. No. 5,344,314 and whichis incorporated herein by reference.

BACKGROUND

The invention relates generally to heating devices that rely on thecombustion of high pressure gaseous fuels such as liquid petroleum gasesor high pressure natural gas, particularly where portability isimportant such as with a heat gun.

Numerous heating applications in home and in industry require a heat gunas a high velocity source of moderately hot air. For example, directheating of objects in processes such as paint removal or shrink wrappingplastics require moderate temperature to avoid charring or overheatingwhile high velocity enhances heat transfer.

A conventional heat gun relies on electrical fans to move the air heatedby combustion of fuel gas. The drawback of these devices is the addedweight of the electrical motors and inability to use these units in thefield where electricity is not available.

A different, commercially highly successful heat gun is described in aprevious patent of Zagoroff, a coinventor of the current invention, U.S.Pat. No. 3,779,694, HEAT GUN, granted Sep. 23, 1973. The fuel jetpressurizes combustion air in a jet pump, feeding the mixture into aninternal combustor. In the combustor, the gases expand by the additionof heat of combustion and exit as a high velocity jet of exhaust gasesat stoichiometric burning temperature of over 3000° F. and dischargevelocities exceeding 4000 feet per minute. This jet entrains and propelscopious amounts of ambient air in a second jet pump to create a blast ofheated air at moderate temperatures. By virtue of the combustion atelevated pressure, a portion of the combustion energy of the fuel isconverted to mechanical energy of the exhaust gases which augments thepower available to pump air in the secondary jet pump.

SUMMARY OF THE INVENTION

The overall mechanical efficiency of the jet pump heat gun to move airsuffers from the inherently low efficiency of the jet pump, typicallyless than 20%. The overall pumping efficiency of the two pumps in seriesis thus only 20%×20%=4%. Another drawback of the device is the greatmixing lengths required for acceptable performance which increases thebulk of the device.

It is therefore an object of his invention to provide a heating devicethat relies on a gaseous fuel as the sole energy source and which ismechanically efficient in converting the heat and kinetic energy of thejet of gaseous fuel to a high speed jet of hot air.

Occasionally, gas turbines built for jet propulsion have been employedfor emergency space heating in the field. In the conventional turbinecycle (see FIG. 1), the energy of the hot exhaust gases of the combustoris used partially to drive a turbine which powers the air compressor,and partially to create the jet of hot air for thrust. The blades of theturbine wheel have to withstand high temperatures and stresses whichnecessitates the use of exotic materials whose cost has precluded thewidespread use of gas turbines for this application.

In accordance with the present invention, a gas heating device such as aheat gun comprises a prime mover such as a turbine which is driven bypressurized gas fuel. An air compressor is driven by the prime mover tocompress air which is then mixed with the pressurized gas fuel andignited in a combustor.

As in the case of the conventional gas turbine, the three maincomponents are a compressor, a turbine and a combustion chamber, butthey are linked together in a novel manner. As before, the compressorfeeds combustion and excess air into the combustion chamber underpressure; the turbine, however, is not driven by the combustion productsbut by the expansion energy of the compressed fuel gas. The turbineblades are thus not exposed to any heat and can be made of inexpensive,low temperature materials.

Preferably, the compressor feeds both the combustion air necessary forcombustion and excess air necessary to dilute the combustion products tothe desired low temperature into the combustion chamber. Combustion andmixing of the excess air are carried out under constant pressure insidethe combustion chamber, and the exhaust gases are expanded through anoutlet nozzle.

In a preferred embodiment of the invention, the compressor furtherdirects some of the compressed air to a downstream mixing region inwhich the air thus directed mixes with the combustion products. Thecompressor includes a manifold that forms upstream vents that providefluid communication with the combustor upstream of the ignition regionand downstream vents that provide fluid communication with thedownstream mixing region. The upstream and downstream vents are sizedrelative to each other to direct more of the compressed air downstreamof the ignition region than upstream thereof.

To maintain a cool exterior, a shroud may be provided about thecombustion chamber with cooling air being drawn between the shroud andthe combustion chamber. The cooling air may be aspirated by the exhaustgases directed through the exhaust nozzle.

Preferably, a circular rim is mounted on radially outward ends ofcompressor fan blades, and the turbine blades extend radially outwardfrom the rim.

The invention is particularly applicable to a heat gun where the heatingdevice is mounted to a handle but may also be implemented for otherapplications such as space heating.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1, is a schematic diagram that illustrates a conventionalgas-turbine cycle.

FIG. 2, is a schematic diagram of the preferred turbine cycle employedin the present invention.

FIG. 3 is partially diagrammatic vertical cross-sectional view of apreferred embodiment of the present invention.

FIG. 4 is a cross-sectional view of the embodiment of FIG. 3 taken alongline 4--4.

FIG. 5 is a front view of the heat gun of FIG. 3.

FIG. 6 is a rear view of the heat gun of FIG. 3.

FIG. 7 is a cross-sectional view of another preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

A novel feature of a heat gun of the present invention is the use of agas driven turbine to compress and mix combustion air with the gas fuelrather than a jet pump. Emergency heating at remote locations has beenprovided by gas turbines built for jet propulsion. For reasons that canbe appreciated by reference to FIG. 1, however, devices based on theconventional turbine cycle are too expensive for widespread heating useon a routine basis.

FIG. 1 shows that exhaust gases from the combustion of fuel and air in acombustor 12 drive a turbine 14. The turbine extracts some of the powerfrom those gases and employs it to drive a compressor 16 which suppliescombustion air to the combustor 12. This arrangement requires that theturbine blades withstand the high temperatures of the exhaust gases.

However, we have recognized that superior performance and smaller sizesuitable to a heat gun can be achieved in a device that employs a cycledepicted schematically in FIG. 2. The cycle that our invention employsis like the conventional gas-turbine cycle in that it employs acompressor 18 to supply combustion air to a combustor 20 and in that thecompressor 18 is driven by a turbine or other prime mover 22 thatextracts power from the fluid flow and converts it to rotational, or"shaft," power. Rather than receiving its power from the combustor'sexhaust gases, however, the turbine is interposed in a flow conduit 24a,24b, that conducts the fuel from a pressurized fuel tank or other source26 to the combustor 20.

Because such an arrangement can employ a compressor of the type that isdriven by rotary power, it does not need to resort to jet pumps and thelong passages that such jet pumps require for effective power transferand pressure restoration. The turbine-compressor combination can be madeseveral times as efficient as a jet pump, moreover, so power extractedfrom the fuel flow can be used to compress not only combustion air butalso the typically much higher volume of cooling air. This eliminatesthe need for the second, exhaust-gas-driven jet pump that the jet-pumpheat gun requires, and the resultant greater achievable pressure canproduce a given mass flow through a gun outlet area that is greater thanis achievable in jet-pump arrangements. A heat gun that employs theteachings of the present invention can therefore be made considerablysmaller than a jet-pump arrangement of the same capacity yet avoid theneed for the exotic materials and resultant cost that conventionalturbine cycles require.

FIGS. 3-6 depict an improved heat gun that embodies the teachings of thepresent invention. Pressurized propane or some other pressurized gaseousfuel from an appropriate source such as a gas canister pressurized to 30psig passes through a nozzle 31, which aims the resulting gas jet atturbine blades 32 mounted on a turbine wheel 33. The turbine wheel 33 isfree to rotate on bearings 34 mounted on a stationary shaft 35. Thisarrangement is commonly known as an axial-flow turbine, and it extractskinetic energy from the gas jet and converts it to shaft power of theturbine wheel. Although we prefer such an arrangement, it will be clearthat the broader teachings of the present invention can be used in a gunthat employs any prime mover that can extract power from the fuel flowand convert it to rotary power.

The turbine wheel 33 carries a set of fan blades 36, which are separatedfrom the turbine blades 32 by a rim 37. As the turbine wheel spins, thefan blades 36 pump air A through a compressor inlet formed by an inletshroud 38. The air flows past shaft-supporting struts 39, and aircompressed by the action of the fan blades 36 is driven into a manifold43 disposed downstream of the fan blades 36 and arranged to distributethe air in a manner that will be described below. Such a compressorarrangement is commonly known as an axial-flow compressor, and itsfunction is to employ the shaft power to maintain as much pumping headas possible on the desired output air volume.

Meanwhile, the turbine discharge gas enters an annular gas plenum 40, atthe downstream end of which is perforated wall 41 that presents enoughflow resistance to cause the gas to assume a uniform radial distributionas it enters a mixing plenum 42, where it is joined by air driventhrough vents 44 in the manifold 43 by the pressure head that thecompressor maintains. The resultant stoichiometric mixture of fuel andair enters a combustion region 46 through flame-holder passages 47,where it is initially ignited by a spark plug 48. The flame-holderpassages 47 are so sized that the velocity of the air passing throughthem exceeds the velocity of flame propagation so that the flame in thecombustion region 46 cannot propagate back into the mixing plenum 42.

In summary of the description so far, the fuel conduit 24a, 24b of FIG.2 is represented in FIG. 3 by the nozzle 31 and annular plenum 40, whilethe prime mover 22 interposed in this conduit is represented in FIG. 3by the turbine. The path from the compressor 18 to the combustor 20 ofFIG. 2 includes vents 44. The heat gun of the present invention is thussimilar to the Zagoroff jet-pump gun mentioned above in that the powerin the expanding gaseous fuel on its way to the combustor is used tomaintain the pressure head necessary to drive the combustion air intothe combustion region with the necessary velocity.

As was stated above, however, the heat gun of the present inventionextracts the power from the fuel-gas flow by means of a turbine (orsimilar prime mover for producing rotary power) rather than by means ofa jet pump, so the combustion air can be pressurized by a type ofcompressor that can be driven by application of rotary power. The energyconversion can therefore be much more efficient than it is when a jetpump is employed, so enough power can be extracted from the fuel-gasflow to maintain a pressure head not only on the combustion air but alsoon the considerably higher volume of air employed to cool the combustionproducts.

Consequently, the combustion chamber includes a mixing duct 49downstream of the combustion region 46, and the manifold 43 additionallyincludes vents 45 by which the compressor directs cooling air A₂ to themixing duct 49. The resultant mixing with the combustion products coolsthem to the desired low temperature. The mixing is enhanced byturbulence that results from a spoiler disk 50 mounted on the manifold43.

To maintain a cool exterior, the heat gun is mounted inside a shroud 52,which is cooled by air A₃ that the exhaust gases E aspirate through theannular passage between an output nozzle 51 and the shroud 52. Thecooling air A₃ is admitted into the annular passage through air passages54, 53, and 55, which are sized and arranged to maintain an evendistribution of cooling air about the nozzle 21. As a further measure,the hot skin of the mixing duct 49 may be wrapped in thermal insulation.

For hand-held operation, a handle 57 may be mounted on the shroud 52 andhouse a gas-inlet fitting 28, a gas shut-off valve 59, a fuel line 60, apiezoelectric igniter 61, a trigger lever 62, and a safety catch 63.

Standoffs 56 are provided to prevent the nozzle 51 from being presseddirectly against an object.

Since the illustrated embodiment enables air compression to be carriedout by means of a blade-type compressor rather than a jet pump, thepower is imparted to the air in a very short axial distance, whereas ajet pump requires a considerable axial distance to be effective.Moreover, since the turbine-compressor combination's much greaterefficiency enables it to use the power from the expanding fuel gas topressurize both the combustion air and the cooling air, it eliminatesthe need for a second jet pump downstream of the combustor and theadditional length that such an additional jet pump requires. And thegreater efficiency also results in development of a greater pressurehead on all of the mixture of exhaust and cooling gas, so the area ofthe exit nozzle for a given flow velocity can be relatively large.

Together, these factors enable a unit that embodies the teachings of thepresent invention to be much smaller than a jet-pump unit of the samecapacity. For example, we have designed a unit in accordance with thefollowing construction parameters:

Overall Length: 11 in.

Overall Diameter: 5 in.

Gas Orifice Size: 0.150 in.×0.040 in.

Outlet Nozzle Size: 0.5 in.×8 in.

Turbine Diameter: 4 in.

Fan Diameter: 3.5 in.

Fuel Gas: Propane

Operating Pressure: 30 psig.

Output Temperature: 1250° F.

Turbine Speed: 20,000 rpm.

Heat capacity: 180 000 BTU/hr.

Turbine Efficiency: 40%

Fan Efficiency: 85%

Compression Ratio: 1.0034

It is clear from the foregoing design parameters that the presentinvention enables a large heating capacity to be achieved in a smallpackage.

It will be apparent to those skilled in the art that the basic principleemployed here can be employed in a wide range of embodiments. Forexample, although the illustrated embodiment employs an axial-flowturbine to extract the power, other prime movers such as a reactionturbine or a vaned or piston "air" motor could be employed instead toproduce the desired rotary power. And the resultant rotary (shaft) powercan be used to drive, for instance, a compressor that comprises acentrifugal- or mixed-flow fan. It is thus apparent that the teachingsof the present invention constitute a significant advance in the art.

FIG. 7 depicts another preferred embodiment of the present invention.Heat device 110 differs from the embodiment depicted in FIGS. 3-6 inthat a baffle 116 is affixed upstream of inlet shroud 38 with a bolt116a and washer 116b to form a radial flow inlet 118 between the shroud38 and baffle 116. As a result, air A enters heating device 110radially. Baffle 116 reduces the noise of heating device 110.

Heating device 110 also includes an air baffle 114 and an air shield112. Air baffle 114 encircles and is spaced away from air shield 112 toform a passageway 120 therebetween. Some of the air A₁ pumped by fanblades 36 is directed by air baffle 114 through vents 122 to mix withgas exiting perforated wall 41 in mixing plenum 42 for combustion. Theremaining air A₂ passes through passageway 120 to mix with combustionproducts in mixing duct 49. Heating device 110 is suitable both as aheat gun, a space heater or a dryer. Additionally, multiple heatingdevices 110 can be mounted to machinery for such purposes as drying andheating products in process as well as shrinking shrinkwrap film.Furthermore, the present invention depicted in FIGS. 3-6 can also beused as a heater or a dryer with the handle being omitted.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A gas heating device comprising a source ofunheated pressurized gas fuel; a housing including means for forming aninner chamber open to the atmosphere at one end for directing flow ofair, means forming an outer annular passage about said inner chamber fordirecting flow of pressurized gas and a combustion chamber for receivingair from the inner chamber and gas from said outer annular passage;Asingle prime mover comprising a first set of radial blades in the innerchamber for compressing air and a second set of radial blades in theouter annular passage and driven only by the expansion energy of theunheated pressurized gas; wherein the pressurized gas fuel is mixed withthe compressed air and ignited in the combustion chamber to formcombustion gases, an exhaust gas nozzle means directing the combustiongases from the combustion chamber.
 2. A gas heating device as claimed inclaim 1 including means directing some of the compressed air to adownstream mixing region in which the air thus directed mixes withcombustion products before being discharged from said nozzle means.
 3. Agas heating device as claimed in claim 1 wherein formed within theexhaust gas nozzle means is surrounded by a shroud through which coolingair is aspirated by exhaust gases directed through the exhaust gasnozzle.
 4. A gas heating device as defined in claim 1 wherein the primemover includes:a stationary shaft; a wheel rotatably mounted on theshaft and mounting the first set of blades which extend radially outwardtherefrom; and a circular rim mounted on radially outward ends of thefan blades and mounting the second set of blades which extend radiallyoutward therefrom.
 5. A gas heating device as claimed in claim 1 whereinthe heating device includes a handle.
 6. A heat gun mounted to a handlecomprising:a source of unheated pressurized gas fuel; a compressorcomprising rotatable fan blades for compressing air into an air baffle;a circular rim mounted on radially outward ends of the fan blades; asingle comprising turbine blades extending radially outward from thecircular rim, the turbine being driven only by the unheated pressurizedgas fuel and driving the fan blades; a combustion chamber downstreamfrom the turbine blades having an exhaust nozzle; a first passage waythrough the baffle for directing a first portion of pressurized air intocommunication with pressurized gas fuel upstream of an ignition regionin the combustion chamber; a second passage way through the baffle fordirecting a second portion of compressed air into the combustion chamberto mix with combustion products; and a shroud, surrounding thecombustion chamber through which air is aspirated by exhaust gasesdirected through the exhaust nozzle.